This invention relates to identification plates and, in particular, to plates which indicate the provenance of an object to which they are attached. Such plates are conventionally termed licence or number plates when attached to a motor vehicle.
From 01 Sep. 2001, each United Kingdom licence plate must conform to British Standard BSAU 145d which governs, inter alia, the impact resistance and the minimum retro-reflectivity of the plate. Similar although, in some cases, less stringent provisions exist across the entire European Union, as they do in most other countries across the world.
GB 2126386 A discloses a method of producing licence plates for a vehicle, in which indicia are printed onto a carrier. The carrier may be either a reflective surface or a plastics coating, such as a relatively thick transparent plastics sheet to provide structural support for the plate.
EP 0806752 A1 discloses an identification plate and a method of manufacture of such a plate wherein a rigid transparent substrate has indicia printed thereon. The resulting printed plate is then adhered to an opaque liner such that the indicia are visible through the transparent sheet.
It is known to print on to a nominally retro-reflective surface and then to adhere to that surface a lens, such as a clear polymer layer, to impart to that surface a retro-reflective character. The lens may be a transparent, relatively thick, acrylic layer applied over the retro-reflective surface or a relatively thin layer with a relatively thick backing layer to provide support, as is disclosed in WO 94/19769 A2.
Retro-reflective sheet material is typically a beaded substance which has a rough surface. Consequently, it is difficult to print on, so special formulations of sheet materials, which are able to ‘take’ the ink, special printing materials such as ribbons, together with the use of elevated print-head temperatures are required. Both of these factors increase the cost. The coarse surface structure of the specially formulated retro-reflective sheet materials can reduce print-head operating life and the elevated print-head temperatures can also reduce print-head lifetimes. It has also been found that print quality suffers at elevated print-head temperatures and with such rough surfaces.